Four elements trainer 0.6.01c2/29/2024 ![]() ![]() the change in ΔR, the effect of the bridge’s nonlinearity from 800Ω of resistance change. In Figure 2, you can see the natural tendency of the bridge’s single-variable element in the form of inherent nonlinearity in its transfer function.įigure 2. For this reason, it’s a good idea, and easier to manage, to have, a common-mode voltage centered around 0V. However, note that changing the common-mode voltage across V AB increases error and complexity in the amplifying second stage, usually realized as a instrumentation quality differential amplifier. The voltage V AB is usually amplified by using a subsequent amplifying stage via a differential amplifier. It’s important to note that this equation also suggests that having a dual supply across the four-legged resistance arrangement could be useful not only to increase the range, but also to help maintain a 0V common-mode voltage across the AB nodes. When R1 = R2 = R3 = R4 = R, the bridge is balanced.įor a single variable-resistance element, for R3 = R + ΔR and R1 = R2 = R4 = R:Įquation 2 suggests that increasing the constant supply voltage, V, to the bridge will increase the output voltage, i.e., the swing range across the bridge. Should a change occur in resistance (ΔR) from R3, then the output differential voltage created is: A typical bridge with nodes A and B sensing output voltage from a change in resistance (ΔR).Īssuming that R1 = R2 = R3 = R4 = R in Figure 1, the bridge is balanced with nodes A and B at a constant V/2 (volts) and with a differential voltage of 0V across V AB. A typical bridge circuit (Figure 1) detects milliohms of changes in resistance (ΔR).įigure 1. Linearity improves by increasing the order of the equation in the microcontroller. For a high-precision system, desiginers have traditionally had to consider both the inherent nonlinearity of the RTD element and the Wheatstone bridge, and then painfully calibrate the front-end while linearizing the front-end at the microcontroller side. RTD devices generally come with very detailed data sheets characterizing their behavior with look-up tables and even transfer function equations down to four or more orders of error compensating terms. By incorporating an RTD element (and based on the RTD manufacturer), the bridge’s inherent resistance variations stay within the accepted linearity and tolerance limits. Using inexpensive, accurate discrete parts, resistance-variable Wheatstone bridge circuits perform most of the front-end tasks in a design. Wheatstone Bridge with Single Variable Resistance Note that when we speak generally about “bridges,” this piece is focused on circuit design for a Wheatstone bridge. In this application note we will take a look at its behavior and explain how you can linearize the bridge circuit to enhance performance. Usually a sensor element, typically a resistance temperature detector (RTD), thermistor, or thermocouple, is situated in the hot/cold environment to provide information about resistance change to temperature. ![]() Bridge circuits are commonly used to detect the temperature of a boiler, chamber, or a process situated hundreds of feet away from the actual circuit. The Wheatstone bridge has a single impedance-variable element that, when away from the balance point, is inherently nonlinear. Since bridge circuit are so simple yet effective, they’re very useful for monitoring temperature, mass, pressure, humidity, light, and other analog properties in industrial and medical applications. Whether the bridges are symmetric or asymmetric, balanced or unbalanced, they allow you to accurately measure an unknown impedance. These long-established circuits are among the first choices for front-end sensors. Wheatstone bridge circuits measure an unknown electrical resistance by balancing two legs-one with the unknown component-of a bridge circuit. The simplicity and effectiveness of a bridge circuit makes it very useful for monitoring temperature, mass, pressure, humidity, light, and other analog properties in industrial and medical applications. We will examine its behavior and explain how to linearize the bridge circuit to optimize performance. This application note discusses the resistance-variable element in a Wheatstone bridge-the first choices for front-end sensors. ![]()
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